Cache

A cache is a store of previously downloaded content kept for reuse, so it doesn't have to be fetched again. Web browsers cache files such as images, stylesheets, and scripts; the website's server tells the browser how long each file may be kept, usually via the Cache-Control , before it must be re-downloaded.

A adds another caching layer between the visitor and your server, which is why clearing a CDN's cache is sometimes necessary to see a change go live.